Try checking RWG Mobile , ScanCom or Mozillion
I bought a SIM card from RWG a while back that was a 1 time payment for 2 years contract for 1GB data per month. It was so cheap I never actually got round to using it.
I believe the other 2 do similar plans.
EDIT -
If you have no luck try searching IOT SIM / Internet Of Things SIM / Prepaid Sim
I can see some offers for 500mb valid for 5 years for £5